11 Perennial Plants You Should Never Divide In The Fall

Dividing plants is an excellent way to rejuvenate and expand your garden. Whether you have plants that are dying or growing leggy, see that your perennials are becoming overcrowded, or simply want to propagate some of your favorite plants for free, division is an easy way to solve these common problems. Although the process is usually quite simple, the thing you need to know about dividing perennials is that certain plants can't or shouldn't be divided in the fall. Generally speaking, the perennials that bloom in the fall should be divided in the spring instead to avoid causing them too much stress.

Aster, certain ornamental grasses, and bearded iris are just a few examples of perennials you should avoid dividing in the fall. In addition to preventing unnecessary stress, spring is the best time to divide certain plants because it gives them plenty of time to establish new roots before winter hits. In these cases, fall division could leave you with dead plants — which no one wants to see.

Suitable for hardiness zones 4 to 8, aster is a gorgeous perennial to grow in your garden for an autumn bloom. The plant produces a star-shaped purple flower, similar in appearance to a daisy. If you notice your plant becoming leggy or dying in the center, it's a good indication that your aster needs to be divided. Division is best done in early spring, when the plant is just leaving its dormancy period. Dividing aster in the fall when it's already in bloom can cause the plant too much stress.

Japanese Sedge (Carex kobomugi Ohwi) is an evergreen ornamental grass variety best suited for hardiness zones 5 through 9. Like most ornamental grasses, Japanese sedge responds best to spring division. Since the grass is evergreen and won't experience a dormancy period, dividing it in the fall may damage the plant, leaving it vulnerable and unlikely to survive the winter. 

Hardy chrysanthemums (Chrysanthemum morifolium), commonly called mums, are a staple of fall. Grown in hardiness zones 5 to 9, mums are beloved for their vibrant colors and long-lasting blooms. Like most perennials that bloom from late summer and into the fall, mums should be divided in the spring. You'll need to wait roughly two years before you start dividing your mums to give them enough time to develop strong roots. Dividing in the fall, when the plant is thriving, can cause the blooms to lose color, or worse, to die altogether.

Bee balm (Monarda didyma) is suitable for hardiness zones 4 through 9. The vibrant flowering perennial is known for attracting bees, hummingbirds, and butterflies. Division is a common way to propagate the plant, but you should do it in spring rather than fall to give the plant plenty of time to establish a healthy root system before it goes dormant for the winter.

Although it is possible to divide larkspur (Delphinium) in the fall, spring division will be gentler on the plant, so it's best to hold off on the procedure until after winter. You can also help avoid transplant shock by watering your larkspur the day before you want to divide it. Larkspur grows in hardiness zones 3 through 9 and is a great option for anyone looking to add color to their late summer or fall garden.

Eulalia grass (Miscanthus sinensis), also known as Chinese silver grass, is a warm-season ornamental grass suitable for hardiness zones 4 through 9. Instead of a traditional root system, this grass has rhizomes, which are technically underground stems. Once mature, these rhizomes are easy to be divided; however, it's best to avoid fall division and wait until the plant is dormant in early spring instead.

Spring and or fall are usually the best seasons for dividing plants, but when it comes to the bearded iris (Iris germanica), summer is your best bet. In the fall (and spring), when conditions are typically damp, you run the risk of the plant's rhizomes developing a fungal disease. Waiting until July or August will offer dryer, safer conditions for the procedure. Summer is also ideal because it gives the rhizomes enough time to establish themselves before winter hits. Available in a range of colors, the bearded iris is suitable for hardiness zones 3 to 10.

It's generally advised to avoid dividing balloon flower (Platycodon grandiflorus) — not just in the fall but year-round. This is because balloon flowers have tap roots that don't hold up well when disturbed. That said, if you have a particularly healthy balloon flower, you can try your hand at dividing, just be prepared for potential disappointment. For the best chance of success, divide in the spring with a plant that's roughly 6 inches tall. These beautiful flowers are ideal for hardiness zones 3 through 8 and get their name from the plump, balloon-shaped buds that develop around summer. 

Ideal for hardiness zones 4 to 9, Jacob's ladder (Polemonium reptans) is a tall perennial that grows up to 24 inches in height and produces cup-shaped flowers. To keep your Jacob's ladder healthy, you will need to divide the plant every three to four years. To make this process easy and stress-free, avoid dividing in the fall and opt, instead, for the spring. Propagation by division is generally easy with this plant, though slicing through the thick root clumps may require a bit of effort.

Maidenhair Fern (Adiantum pedatum) grows best in hardiness zones 3 to 11 and produces lush, arching greenery. Whether you're growing it indoors or outdoors, it's best to divide maidenhair ferns in the spring to give them a chance to establish their roots well before the cold weather returns at the end of the season. The roots of the plant are quite tough, so a serrated knife will work best for separating them. 

Hardy in zones 4 through 9, toad lilies (Tricyrtis hirta) take well to propagation by division. Since this plant blooms in the late summer and fall, dividing it is best saved for spring. Propagating the toad lily in this way is pretty straight forward. Simply dig around the plant, lift its roots carefully out of the ground, and use garden snips to create several healthy clumps.